This invention relates generally to lithium-sulfur batteries, and in particular to lithium electrodes having protective dioxolane coatings.
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries. The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign secondary battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as, for electric vehicles and load leveling. Of course, such batteries are also suitable for use in lower power applications such as cameras or portable recording devices.
At this time, the most common secondary batteries are probably the lead-acid batteries used in automobiles. These batteries have the advantage of being capable of operating for many charge cycles without significant loss of performance. However, such batteries have a low energy to weight ratio. Similar limitations are found in most other systems, such as Ni--Cd and nickel metal hydride systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery has been sought for the next generation of batteries. The low equivalent weight of lithium renders it attractive as a battery electrode component for improving weight ratios. Lithium provides also greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
Recent developments in ambient-temperature sulfur electrode technology may provide commercially viable rechargeable lithium-sulfur batteries. Chu and colleagues are largely responsible for these developments which are described in U.S. Pat. Nos. 5,582,623 and 5,523,179 (issued to Chu). The patents disclose a sulfur-based positive electrode for a battery cell that has low equivalent weight and high cell voltage and consequently a high specific energy (greater than about 120 Wh/kg). The disclosed positive electrode addresses deficiencies in the prior art to provide a high capacity sulfur-based positive composite electrode suitable for use with metal (such as lithium) negative electrodes in secondary battery cells. These developments allow electrochemical utilization of elemental sulfur at levels of 50% and higher over multiple cycles. Because sulfur has a theoretical maximum capacity of 1675 mAh/g (assuming all sulfur atoms in an electrode are fully reduced during discharge), the utilization of sulfur in lithium-sulfur cells as described in the above Chu patents typically exceeds 800 milliamp-hours per gram (mAh/g) of sulfur.
The lithium-sulfur batteries described in the above Chu patents provide increased capacity (have a high specific energy) relative to previously available battery cells, such as the above-noted lead-acid batteries. However, like previous battery designs, they are susceptible to the reaction of the lithium electrode with the electrolyte and/or dissolved sulfur species. This leads to several problems including low cycling efficiency of lithium and the possibility of "pillowing". Low cycling efficiency of lithium is problematic in that it necessitates the use of excess lithium in the cell to make up for that is lost on each cycle. This adds weight and volume to the cell that it undesirable. Lithium-sulfur battery cells are typically composed of lithium and sulfur electrodes and an electrolyte (liquid, gel or solid) sealed in a casing. Pillowing occurs when gas is produced as a result of a chemical reaction within the cell. The increased volume occupied by such a gas relative to the solid and/or liquid phase battery cell components from which it is derived increases the pressure within the sealed casing, causing it to bulge, or "pillow."
An example of this situation is illustrated in FIGS. 1A and 1B. The figures show representations of a lithium-sulfur secondary battery cell 100. The cell's electrodes and electrolyte are sealed within a flexible pouch-like casing 102. In a normal cell which functions properly, illustrated in FIG. 1A, the casing 102 roughly conforms to its contents. FIG. 1B shows a representation of the battery cell 100 after it has pillowed. As noted above, the pillowed cell results from a build up of pressure within the casing 102 due to unintended and undesirable gas formation within the casing 102. The casing of a pillowed cell no longer roughly conforms to its contents, but instead bulges outward.
Pillowing is undesirable in battery cells for a number of reasons. First, pillowing suggests an unintended reaction within the cell that is likely to detrimentally impact its performance. Second, the deformation of the cell due to pillowing, particularly its increase in size, may result in the cell no longer fitting in its allotted compartment in the electronic device which it is intended to power. Thirdly, the increased internal pressure which causes pillowing may cause the casing of the cell to burst, allowing access of contaminants into the cell, and escape of lithium and other potentially dangerous battery cell components from the cell. Pillowing is typically observed in secondary battery cells, however it may also occur in primary battery cells with similar detrimental consequences.
Accordingly, lithium-sulfur battery cell designs which mitigate pillowing would thus be desirable.